In version 3.4, MongoDB removes support for SCCC config servers.
Before you can upgrade your sharded clusters to 3.4, you must
convert your config servers from SCCC to CSRS. To convert to CSRS,
follow the procedure to change your 3.2.x SCCC to 3.2.x CSRS.

The procedure outlined in this tutorial requires
downtime. If all the sharded cluster binaries are at least version
3.2.4, you can also convert the config servers to replica set without
downtime. For details, see Upgrade Config Servers to Replica Set.

Start the new mongod instances to add to the replica set.
These instances must use the WiredTiger
storage engine. Starting in 3.2, the default storage engine is
WiredTiger for new mongod instances with new data paths.

Important

Do not add existing config servers to the replica set.

Use new dbpaths for the new instances.

The number of new mongod instances to add depends on the
config server currently in the single-member replica set: